Hello Bishnu,
first point is not clear to me .
for second point once the PO is created for plant 5200, there you can maintain output type for medium 6 as EDI to send the IDOC to merchand . No need to use the workflow for intimating to merchant . the IDOC will create the SO in merchant system once the PO for plant 5200 is saved .
Hope this information help you .
Thanks,
Tausif